1. Field of the Invention
This invention relates to measuring instruments responsive to fluid pressure. More particularly, this invention relates to such instruments of the type utilizing a vibratable-wire as the basic force-sensing element.
2. Description of the Prior Art
Instrumentation systems for use with industrial processes have for many years utilized apparatus of various kinds for measuring fluid pressures, especially differential pressures such as are produced across an orifice plate in a flow pipe for the purpose of developing a fluid flow-rate signal. Commonly such apparatus has comprised a differential-pressure cell of the force-balance type, such as shown in U.S. Pat. No. 3,564,923. Although such instruments have performed admirably over a number of years, it has become evident that the conventional devices do not fully meet the needs of modern industrial process instrumentation systems. For example, modern systems have a considerably greater need for stability and accuracy over widely varying conditions, such as temperature, etc., than can be met by conventional prior art equipment.
It had long ago been proposed by workers in the measuring instrument art to use a vibratable-wire as the basic sensing element for pressure-responsive instruments. Such proposals were based on the understanding that the frequency of wire vibration was closely related to the tension in the wire, and the recognition that the wire tension could in turn be controlled by a differential pressure to be measured. Thus, the frequency of wire vibration could be developed as a measurement signal responsive to differential pressure. Moreover, theoretical considerations indicated that such an instrument could be highly accurate in performance.
There are many prior patent disclosures of vibratable-wire devices, and simply as an indication of several types of such prior art, reference may be made to U.S. Pat. Nos. 2,447,817; 2,455,021; and 3,047,789. U.S. Pat. No. 3,393,565 describes several vibratable-wire instruments intended for measuring differential pressure, and shows in FIG. 2 a device with the wire in a liquid-filled container. The structure of FIG. 2 includes a pivoted lever 26 for coupling an input force to the wire, and in column 3, at line 15 et seq. there is suggested a modification wherein the lever is eliminated and the wire is secured directly to one of the diaphragms.
The proposed instruments resulting from such early work with vibratable-wire sensors have, however, not found acceptance in the art of process instrumentation, primarily because they have not been capable of adequate performance under required operating conditions. Such earlier proposals thus did not provide practical solutions to the problem, and did not advance the state of the art to the point where the results achieved were useful to the process industries.
A more recent development in the art of pressure-responsive instruments has resulted in an accurate and practical vibrating-wire instrument for measuring differential gas pressures. A description of that instrument is set forth in copending application Ser. No. 732,130, filed by E. O. Olsen et al on Oct. 13, 1976, now U.S. Pat. No. 4,149,422, patented Apr. 17, 1979. An electronic transmitter adapted for use with such a vibrating-wire instrument is disclosed in copending application Ser. No. 732,129, filed E. O. Olsen et al on Oct. 13, 1976, now U.S. Pat. No. 4,118,977 patented Oct. 10, 1978.
Although the differential-gas-pressure instrument referred to above has performance abilities suited for its intended purpose, it does not have the capability of serving as a general purpose differential-pressure instrument for industrial processes. Specifically, that apparatus is not capable of measuring the various differential liquid pressures which must be measured in most commercial processes of the present day.